1) Field of the Invention
This invention relates to a picture color tone controlling method and apparatus for a printing press, and more particularly to a picture color tone controlling method and apparatus for controlling the color tone using an IRGB densitometer.
2) Description of the Related Art
As one of conventionally available methods of controlling the color tone of a picture of a printing press, a method is known wherein a color patch for color tone inspection is printed at a marginal portion of a printing sheet together with a picture and a spectral reflection factor of the color patch is measured using a spectrometer and then a displacement of the color tone of the picture from a target color tone based on a result of the measurement to control the ink supply amount for each color. However, since this method requires a margin for printing a color patch on a printing sheet, the paper is consumed wastefully by an amount of the margin.
In order to solve this problem, a method is proposed in Japanese Patent Laid-Open No. 2001-18364 (hereinafter referred to as Patent Document 1) or Japanese Patent Laid-Open No. 2001-47605 (hereinafter referred to as Patent Document 2) wherein picture color tone control of a picture itself is performed without using a color patch. The methods disclosed in the documents mentioned generally adopt the following procedure.
First, the spectral reflection factor of a picture printed by printing units for individual colors is measured using a spectrometer. Then, the spectral reflection factor (average spectral reflection factor of an entire key zone) is arithmetically operated for each of key zones of ink keys, and the spectral reflection factor of each key zone is converted into color coordinate value s (L*a*b*) proposed by the Commission Internationale de l'Eclairage (CIE). The ink supply amount for each color is adjusted to perform test printing, and if a printing sheet having a desired color tone (such sheet is hereinafter referred to as OK sheet) is obtained, then the color coordinate value of each key zone of the OK sheet is set to a target color coordinate value. Then, regular printing is started, and the difference (color difference) in color coordinate value between the OK sheet and a printing sheet (in the following description, a printing sheet obtained by the regular printing is referred to as regular printing sheet) is calculated for each key zone. Then, the increase/decrease amount of the opening of the ink key of each printing unit with respect to the color difference is calculated, and the opening of the ink key of each printing unit is adjusted by on-line control so that the color difference may be zero.
However, the method disclosed in Patent Document 1 or 2 has the following problems. First, according to the method described above, while a spectrometer is used as a measuring instrument, the spectrometer requires a high cost, and besides, when the object of measurement moves at a very high speed as in a rotary press for newspapers (in this instance, the object of measurement is a printing sheet), the spectrometer cannot follow up the object of measurement from the processing capacity thereof. Further, according to the method described above, since color tone control is started after an OK sheet is printed, many loss papers are produced before an OK sheet is printed after printing is started. Furthermore, according to the method described above, since the spectral reflection factor of a picture in a key zone of an ink key is averaged over the entire key zone and the color tone control is performed based on the average spectral reflection factor, where the image line rate of the picture in the key zone is low, the error in measurement of the spectrometer becomes so great that the control is liable to become less stable. Besides, although, depending upon an order of a customer, particularly strict color tone management is required for a particular noticed point in the picture, where it is intended to perform color tone control with regard to a particular noticed point in this manner, data of the PPF (Print Production Format) of the CIP3 (Cooperation for Integration of Prepress, Press, Postpress) or the like must be received as reference image data from a plate making step on the upstream.